In the final battle between John Matrix and Bennett a voice (that of Director Mark L. Lester most likely) can be heard giving directions to the actors: "Switch!"

This voice command can clearly be heard when Matrix (Arnold Schwarzenegger) is being backed into some fire by Bennett (Vernon Wells), while fighting inside of the boiler room.

Just as Matrix is about to turn the tables around/switch places with Bennett, so that the crazed villain can feel the heat, a voice can be heard over the burning fire and grunts of the actors saying: "Switch!".

Put up the volume a bit higher if you must, but your ears will not be deceiving you, trust me!

This was obviously done so that the actors would know exactly when to switch places with each other. However, the director's voice should have never been heard in the actual movie, that is a major slip-up!